Many variations of wrap spring clutch systems for the operation of window shades have been developed and are familiar to those skilled in the art. These clutch systems typically employ a fixed cylindrical core shaft or stud about which a helical coil spring is positioned, a rotary drive member capable of controlling the inner diameter of the coil spring, and a bushing adapted to ensure engagement of the shaft, spring, and rotary members. During operation of the typical wrap spring system, the rotation of the rotary drive member increases the inner diameter of the wrap spring, which reduces the friction between the core shaft and wrap spring, thereby allowing the lowering or raising of the window shade as desired.
U.S. Pat. No. 4,779,662 issued to Wilk on Oct. 25, 1988 is a typical example of a window shade wrap spring clutch systems found in the prior art. The Wilk patent discloses a stationary stud upon which is wound a wrap spring, wherein the wrap spring normally hugs or binds against the stud. A drive member in the form of a pulley is positioned around the stud and wrap spring, such that, once the pulley is rotated, the pulley inner bore can be placed in contact with one radially-extended tang of the wrap spring. A "tang" is the end of the wrap spring extending radially from the circumference of the spring. During operation, the pulley is rotated manually by means of a ball-chain. The manual rotation of the pulley causes the pulley inner bore to contact a first wrap spring tang. Continued rotation of the pulley separates the spring tangs, thereby enlarging the effective inside diameter of the spring, allowing the spring to slip and rotate about the core shaft. The portion of the wrap spring opposite the contacted tang slides along the core shaft member creating a friction drag, which serves to stabilize the raising and lowering of the window shade during operation.
While the friction drag helps to smooth the movement of the wrap spring clutch system, it usually does not eliminate the jerky motion, which is often inherent in the operation of wrap spring clutch systems. This inherent jerky motion typically occurs when the clutch system is operated in "overrunning" mode, wherein the weight of an overrunning load, such as a shade, torques the drive member in the same direction as the torque being applied by the drive member. This overrunning load can cause the system wrap spring to intermittently store and release energy during the operation of the clutch. This intermittent energy transfer produces a jerky motion that is sometimes called "stair-stepping," during which the overrunning load drops by a small, finite distance then halts, then resumes dropping (analogous to the motion of an object descending a flight of stairs).
Previous attempts at reducing the effect of stair-stepping in wrap spring clutch systems have met with little success. For example, U.S. Pat. No. 4,433,765, issued Feb. 28, 1984 to E. Rude et al., attempts to eliminate stair-stepping by teaching a spring clutch system which uses multiple springs disposed between two coaxial mounted cylindrical elements, wherein the springs are designed to slip so that all of the springs support the overrunning load. Effective operation of the Rude patent depends on the designer's ability to determine the effective slip torque between the spring and the core shaft, where the slip torque value varies with the interference between the outside diameter of the shaft to the inside diameter of the relaxed spring to core shaft diameter ratio. However, the Rude patent is limited in that it ignores the difficulty in controlling spring tolerances in view of the various annealing and plating procedures typically required to avoid corrosion. Annealing and plating of metal springs often results in a lack of uniformity in spring dimension and absolute stiffness, from unit to unit. This lack of spring uniformity makes it extremely difficult to calculate the correct slip torque needed to ensure that the system will operate within design parameters. Consequently, the springs of the multiple spring system seldom open according to designer calculations. This unpredictable spring performance often causes the springs to grab unevenly which typically increases the inherent stair-stepping condition.
The prior art also reveals an attempt by inventors to use nonmetallic wrap springs in the wrap spring clutch design in an effort to eliminate the stair-stepping effect. For example, U.S. Pat. No. 5,669,432, issued Sep. 23, 1997, to J. Nisenson et al., teaches away from using metal in the construction of the wrap spring clutch system. To aid in eliminating the clutch system stair-stepping effect, the Nisenson patent uses molded plastic in the construction of the core shaft and the wrap spring members. The Nisenson patent seeks to eliminate the stair-stepping effect, by controlling the chemical composition of both the wrap spring and the stud in an attempt to improve the frictional engagement between the spring and stud members. Nisenson teaches incorporating in the plastic specific chemical additives that can effect the plastic's physical characteristics, such as stiffness, fatigue strength, wear, stretch and compression coefficients. The Nisenson patent is limited in that it uses a plastic wrap spring construction which is typically not as rugged and reliable over extended periods of operation as the clutch systems using metal springs. This lack of ruggedness and reliability is inherent in plastic springs and often leads to an increase in the relaxed inside diameter of the spring over extended usage, which, in turn, commonly increases the stair-stepping effect.
Therefore, there is a need for a wrap spring clutch system with diminished stair-stepping effect, wherein the clutch system uses a single spring and is therefore not dependant on wrap spring uniformity and tolerances. A need also exists for a wrap spring clutch system with improved frictional engagement between the wrap spring and stud members, and which also has the ruggedness and reliability necessary to withstand extended use.